1. Field of the Invention
The present invention relates to a semiconductor sensor for an accelerometer and to a method for making the same.
2. Description of the Related Art
A semiconductor sensor for an accelerometer is known having a thin beam portion on which is mounted a strain sensing device. This detects acceleration with a high sensitivity.
When a high sensitivity is attained in this way and excessive acceleration is applied, however, this beam portion may sometimes be broken due to the force applied to it.
To avoid this problem, as shown in FIG. 1, there has been provided another semiconductor sensor in which a stopper 1 is provided to restrict the amount of displacement of the beam portion 1 when it receives an accelerating force.
In FIG. 1, B denotes a beam portion consisting of a thin beam portion 3 and a thick beam portion 2.
In this construction, when the thick beam portion 2 is displaced, for example, in an upper direction due to acceleration, the thin beam portion 3, which supports the thick beam portion 2 is deformed by the stress applied thereto in response to the displacement.
A piezo resistance device 6 is provided on the thin beam portion 3 as a stress sensing device and thus the amount of the acceleration applied to the sensor can be detected by the change of the resistance value generated in response to the deformation of the thin beam portion 3.
Also, when the above sensor receives excessive acceleration causing the thick beam portion 2 to be deformed by a great amount, the displacement thereof is stopped by a stopper 1.
In the construction of the known semiconductor sensor for an accelerometer mentioned above, however, when the thin beam portion 3 is formed extremely thin in order to detect acceleration with a high sensitivity and when the sensor receives excessive acceleration, the thick beam portion 3 is displaced upwardly to contact the stopper 1, whereby the movement of the thick beam portion 2 is stopped by the stopper contacting the edge of the thick beam portion 2.
In this situation, since the contacting portion between the stopper 1, provided near one of the edges of the thick beam portion 2, and the edge of the thick beam portion 2 serves as a fulcrum, an inertial moment defined by the distance between the fulcrum and a center of gravity of the thick beam portion 2 and a weight thereof, acts on the thick beam portion 2 and thus it tends to continue moving utilizing the center of gravity as a dynamic point and also tends to displace around the fulcrum point.
Accordingly, the opposite end of the thick beam portion 2 of the beam portion B, i.e., the thin beam portion 3, becomes the working point and thereby the thick beam portion 2 is displaced upward causing a strong stress on the thin beam portion. Eventually, the thin portion of the thin beam portion 2 may be broken.